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Origins of the Universe The Greeks Ptolemy and Aristotle Planets were believed to be perfect spheres and orbits were circles Earth was the centre of the universe They could track the motions of the sun, moon, Mercury, Venus, Jupiter, Mars, and Saturn Geocentric View of Universe Ptolemy & Aristotle Ptolemy & Aristotle “GEO” = Earth “Centric” = center Problems with the geocentric model: Retrograde motion: during certain times of the year Mars, Jupiter and Saturn appeared to be moving backwards Problems with the geocentric model Ptolemy explained this with epicycles – planets went in little circles as they orbited. Heliocentric View of Universe Copernicus “HELIO” = Sun “Centric” = center Orbits are still circles The Heliocentric Model Provided a simpler way to explain retrograde motion Galileo Further added support to the heliocentric model when he made his first observations with a telescope Galileo was able to observe craters on the moon Sun spots that changed over time suggesting the rotation of the Sun Moons that orbited around Jupiter (which also contradicted the geocentric model) Stars were farther away than planets Kepler Used careful measurements and star charts recorded by Tycho Brahe His calculations showed that the planetary orbits were elliptical (NOT circular) So why did the planets orbit in ellipses, not circles? Newton Law of Universal Gravitation – there is a force between all objects that pulls them together Larger objects have more gravity Gravity explained why the planets moved in elliptical orbits – further supporting the heliocentric model. Exploring Elliptical Orbits Activity • Kepler – established that the planets moved around the sun in elliptical orbits. Edwin Hubble observed that the galaxies were moving away from each other He predicted there was a time when all matter was together These observations led to the BIG BANG THEORY Our Solar System INNER/TERRESTRIAL PLANETS OUTER/GASEOUS/JOVIAN PLANETS Planets Are different based on: Size Motion Composition Density Surface temperature